Local preferred direction routing

ABSTRACT

Some embodiments of the invention provide a method for routing. The method defines at least one wiring layer that has at least two regions with different local preferred wiring directions. The method then uses the differing local preferred wiring directions to define a global route on the wiring layer. The two regions are a first region with a first local preferred wiring direction, and a second region with a second local preferred wiring direction. The global route traverses the first region along the first local preferred wiring direction and traverses the second region along the second local preferred wiring direction.

CLAIM OF BENEFIT TO PRIOR PROVISIONAL APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 60/577,434, filed on Jun. 4, 2004.

BACKGROUND OF THE INVENTION

An integrated circuit (“IC”) is a semiconductor device that includesmany electronic components (e.g., transistors, resistors, diodes, etc.).These components are often interconnected to form multiple circuitcomponents (e.g., gates, cells, memory units, arithmetic units,controllers, decoders, etc.) on the IC. An IC also includes multiplelayers of metal and/or polysilicon wiring that interconnect itselectronic and circuit components. For instance, many ICs are currentlyfabricated with five metal layers. In theory, the wiring on the metallayers can be all-angle wiring (i.e., the wiring can be in any arbitrarydirection). Such all-angle wiring is commonly referred to as Euclideanwiring. In practice, however, each metal layer typically has one globalpreferred wiring direction, and the preferred direction alternatesbetween successive metal layers.

Many ICs use the Manhattan wiring model that specifies alternatinglayers of horizontal and vertical preferred direction wiring. In thiswiring model, the majority of the wires can only make 90° turns.Occasional diagonal jogs are sometimes allowed on the preferredhorizontal and vertical layers. Standard routing algorithms heavilypenalize these diagonal jogs (i.e. assess proportionally highrouting-costs), however, because they violate the design rules of theManhattan wiring model. Some have recently proposed ICs that use adiagonal wiring model to provide design rules that do not penalizediagonal interconnect lines (wiring). Interconnect lines are considered“diagonal” if they form an angle other than zero or ninety degrees withrespect to the layout boundary of the IC. Typically however, diagonalwiring consists of wires deposed at ±45 degrees.

Typical Manhattan and diagonal wiring models specify one preferreddirection for each wiring layer. Design difficulties arise when routingalong a layer's preferred direction because of obstacles on these wiringlayers. For example, design layouts often contain circuit components,pre-designed circuit blocks, and other obstacles to routing on a layer.Such obstacles may cause regions on a layer to become essentiallyunusable for routing along the layer's single preferred direction.

An example that shows obstacles that cause regions on a design layout tobecome unusable for routing is illustrated in FIG. 1. This figure showstwo wiring layers that each have two routing obstacles 115 and 120. Oneof the layers has a horizontal preferred direction; the other layer hasa diagonal preferred direction. The obstacles 115 and 120 cause tworegions 105 and 110 to become unusable for routing on both of theselayers. Therefore, both the Manhattan and diagonal wiring modelstypically waste routing resources on the layers of a design layout.

Accordingly, there is a need in the art for a wiring model that allowsManhattan and diagonal wiring and recaptures the routing resources lostbecause of obstacles on a wiring layer. More generally, there is a needfor a route planning method that maximizes the routing resources on eachparticular layer.

SUMMARY OF THE INVENTION

Some embodiments of the invention provide a method for routing. Themethod defines at least one wiring layer that has at least two regionswith different local preferred wiring directions. The method then usesthe differing local preferred wiring directions to define a global routeon the wiring layer. The two regions are a first region with a firstlocal preferred wiring direction, and a second region with a secondlocal preferred wiring direction. The global route traverses the firstregion along the first local preferred wiring direction and traversesthe second region along the second local preferred wiring direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth in the appendedclaims. However, for purpose of explanation, several embodiments of theinvention are set forth in the following figures.

FIG. 1 illustraes an example that shows obstacles that cause regions ona design layout to become unusable for routing is illustrated in.

FIG. 2 illustrates an example of a design layout with severaldifferently shaped local preferred direction (LPD) regions according tosome embodiments of the invention.

FIG. 3 illustrates another example of a wiring layer with severaldifferently shaped LPD regions (LPDRs) according to some embodiments ofthe invention.

FIGS. 4A and 4B provide examples that illustrate the advantage of LPDwiring model in allowing routing resources normally lost due toobstacles on a wiring layer to be recovered.

FIG. 5 illustrates an example of such a joining model of someembodiments.

FIG. 6 illustrates another example of a joining model.

FIGS. 7 and 8 illustrate examples of regions that are between macros orbetween macros and the layout boundary and that would benefit from LPDwiring.

FIG. 9 illustrates an auto-LPDR generation process that is used by someembodiments to generate LPDRs in a layout.

FIG. 10 illustrates an example of a decomposition operation on a layerwith a horizontal global preferred direction.

FIG. 11 illustrates a 45° LPD for an LPDR that is defined between twomacro blocks that are diagonally offset from each other.

FIG. 12 illustrates the elimination of several candidate LPD regions,which were created in the tessellation illustrated in FIG. 10.

FIG. 13 presents an example of an LPDR that is created on a layerbetween two macros.

FIG. 14 illustrates an example of a pin adjustment operation thatmodifies the shape of an LPDR.

FIG. 15 illustrates an example that shows the use of the corridorcreated by the adjustment for a via access to the pin. For thismodification,

FIG. 16 illustrates an example of how some embodiments allow access tothe pin through a 45° jog into the modified LPDR, a vertical traversalthrough this LPDR, and then traversing back in the 45° direction afterleaving the LPDR.

FIG. 17 illustrates another example of the pin adjustment operation.

FIGS. 18-21 illustrate examples of impermeable boundaries betweenregions on a layer and examples of eliminating such boundaries byreshaping the regions.

FIG. 22 illustrates an example of a boundary adjustment operation.

FIGS. 23-25 illustrate examples extensions of LPDRs.

FIG. 26 illustrates a process that the LPDR generator of someembodiments uses to create LPDRs between power via arrays on diagonalwiring layers.

FIG. 27 illustrates an example of creating LPDRs about power structures.

FIG. 28 illustrates the merging of the two LPDRs and to define a newLPDR.

FIG. 29 illustrates an example of an alternative embodiment of theinvention.

FIGS. 30-32 illustrate small sections of congestion and length grids.

FIGS. 33-41 illustrate several examples of planar edges within therouting grids.

FIG. 42 illustrates an example of the planar congestion edges that areused to measure congestion along a layer for the example illustrated inFIG. 41.

FIG. 43 illustrates a process that conceptually represents the overallflow of the router in some embodiments of the invention.

FIG. 44 illustrates a path search that starts at a node and ends at anode.

FIG. 45 illustrates several examples of edge regions.

FIG. 46 illustrates two capacity tiles used in some embodiments of theinvention.

FIG. 47A and 47B illustrate examples of pixelating a capacity tile intonumerous tiles with pixels at their centers.

FIG. 48 presents an example that illustrates how some embodiments definethe pixel pitch and the track pitch.

FIG. 49 presents an example that illustrates that a diagonal movementfrom a first pixel to a second pixel require not only that the secondpixel be free but also requires the two pixels that neighbor both thefirst and second pixels to be free.

FIGS. 50-56 illustrate examples that illustrate how some embodimentsdefine the capacity of edges for defining routes.

FIG. 57 conceptually illustrates a computer system with which someembodiment of the invention are implemented.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous details are set forth for purposeof explanation. However, one of ordinary skill in the art will realizethat the invention may be practiced without the use of these specificdetails. In other instances, well-known structures and devices are shownin block diagram form in order not to obscure the description of theinvention with unnecessary detail.

Some embodiments of the invention provide one or more Electronic DesignAutomation (EDA) tools that use a Local Preferred Direction (LPD) wiringmodel. An LPD wiring model allows at least one wiring layer to haveseveral different local preferred directions in several differentregions of the wiring layer.

Several features of LPD design will be discussed below. Section Iprovides examples of LPD wiring models of some embodiments of theinvention. Section II then describes how some embodiments define regionswith different LPDs on the same wiring layer. Section III then describesglobal routers that use the LPD wiring model, while section IV describesdetailed routers that use the LPD wiring model.

I. LPD Overview

A. Definitions

Several embodiments of the invention provide a router that routes a setof nets in a region of an integrated circuit (“IC”) layout. Each routednet includes a set of routable elements in the IC-layout region. Theroutable elements are pins in the embodiments described below, althoughthey might be other elements in other embodiments. The routes defined bysome embodiments have “diagonal” edges. In some embodiments, a diagonaledge typically forms an angle other than 0° or 90° with respect to thelayout's Cartesian coordinate axes, which are often parallel with thelayout's boundary and/or the boundary of the layout's expected IC. Onthe other hand, a horizontal or vertical edge typically forms an angleof 0° or 90° with respect to one of the coordinate axes of the layout.The horizontal and vertical directions are referred to as the Manhattandirections.

Given a design layout with routing layers, some embodiments describe thewiring model of a layout in terms of (1) several wiring layers, (2) aglobal preferred direction D_(L) for each layer L, and (3) apotentially-empty set of LPDs for each wiring layer L. Some embodimentsdefine a “preferred” direction as the direction that a majority of thewires are laid out in a region. Some embodiments further quantify thisamount in terms of percentages or amount of the wiring. For example,some embodiments define the preferred direction of a layer as thedirection for at least 50% of the wires (also called interconnect linesor route segments) on the layer. Other embodiments define the preferreddirection of a layer as the direction for at least 1000 wires on thelayer.

Some embodiments of the invention use a five-layer wiring model thatspecifies the following global preferred directions: horizontal wiringon wiring layer 1, vertical wiring on wiring layer 2, horizontal wiringon wiring layer 3, +45° diagonal wiring on wiring layer 4, and −45°diagonal wiring (also referred to as 135° or D135 wiring) on wiringlayer 5. One of ordinary skill will realize that other embodimentsspecify the global wiring directions differently or use a differentnumber of wiring layers.

On a particular layer, a region is called an LPD region (or an LPDR)when the region has a local preferred wiring direction that is differentthan the global preferred wiring direction of the particular layer. Inaddition to the global preferred direction D_(L), some embodimentsdefine for each wiring layer L (1) at least 4 pitch values for usewhenever a global or local preferred direction can be 0°, 45°, 90°,135°; and (2) a possibly empty set of data tuples that represent regionson the layer that might have a local preferred direction that differsfrom the global preferred direction D_(L) of the layer.

The pitch values describe the track pitch along a global or localpreferred direction. In some embodiments, pitch values may change fromlayer to layer. Also, in some embodiments, each region's particulartuple t includes an “octangle” O_(t) that represents the shape of theparticular region, and a direction d, that represents the localpreferred direction (i.e., 0°, 45°, 90°, 135°) of the particular region.Some embodiments allow a region's LPD d_(t) to be the same direction asthe global one.

An octangle in some embodiments is a data structure that is useful fordesign layouts that have items with horizontal, vertical, and/or ±45°directions. Specifically, in these embodiments, an octangle represents aconvex geometric shape in terms of eight values, x_(LO), y_(LO), s_(LO),t_(LO), X_(HI), y_(HI), s_(HI), and t_(HI). These eight values defineeight half-planes in two coordinate systems, where one coordinate systemis a Manhattan coordinate system that is formed by an x-axis and ay-axis, and the other coordinate system is a 45°-rotated coordinatesystem that is formed by an s-axis and a t-axis. The s-axis is at a 45°counterclockwise rotation from the x-axis, while the t-axis is at a 135°counterclockwise rotation from the x-axis. In the layouts of someembodiments, horizontal lines are aligned with the x-axis, verticallines are aligned with the y-axis, 45° diagonal lines are aligned withthe s-axis, and −45° diagonal lines are aligned with the t-axis.

Octangles are further described in U.S. patent application Ser. No.10/443,595 entitled “Method and Apparatus for Representing Items in aDesign Layout,” which published as U.S. Published patent application2004-0225983A1. This patent application is incorporated herein byreference. In the description below, both the wiring and non-wiringgeometries of the design layout are convex shapes, or can be decomposedinto convex shapes, that have horizontal, vertical, and +45° sides. Oneof ordinary skill will realize, however, that some embodiments might usethe octangle data structure in cases where the wiring or non-wiringgeometries are more restricted.

Some embodiments impose several consistency requirements on an LPDdescription. For instance, some embodiments require each LPD region tobe entirely within the chip area. Also, in some embodiments, differentLPD regions on a given layer can abut only at their boundary. Inaddition, in some embodiments, all LPD regions are non-degenerate, i.e.they contain at least one interior point.

Careless definition of LPDs can lead to curious consequences like aseparated island on a plane that allows almost no wiring to enter orleave. Since EDA tools typically provide no intelligence about theintention or suitability of such a description, some embodiments of theinvention implement an additional plausibility analysis as a separatechecking stage that can be called from a Graphical User Interface (GUI)or a text-based interface (such as the Python Interface) in an initialplanning stage.

A macro block is a complex pre-designed circuit block that is used in alayout. Examples of such blocks include IP Blocks, RAM cells, etc.

B. Examples

An example of a design layout with several differently shaped LPDregions according to some embodiments of the invention is illustrated inFIG. 2. This example shows a wiring layer 200 having a 45° globalpreferred direction, an octagonal region 205 having a −45° localpreferred direction, an octagonal region 210 having a horizontal (0°)local preferred direction, and a rectangular region 215 having avertical (90°) local preferred direction.

FIG. 3 illustrates another example of a wiring layer with severaldifferently shaped LPD regions according to some embodiments of theinvention. This example shows a wiring layer 300 having a 90° globalpreferred direction. The layer 300 has four LPD regions having differentshapes and different local preferred directions. In the center of thelayer 300 is an octagonal LPD region 305 having a −45° local preferreddirection. Adjacent to the lower-left side of region 305 is arectangular LPD region 310 having a 45° local preferred direction.Adjacent to the upper-left side of region 305 is a hexagonal LPD region315 having a 60° local preferred direction. Adjacent to the right sideof region 305 is a square LPD region 320 having a horizontal (0°) localpreferred direction. This example illustrates the flexibility of the LPDwiring model when designing a wiring layer with different shaped LPDregions having different local preferred directions.

The examples illustrated in FIGS. 2 and 3 present simple cases of theLPD wiring model in a design layout. However, these examples do notillustrate any macros or other obstacles to the wiring on a layer. Oneof the advantages of the LPD wiring model is that it allows routingresources normally lost due to obstacles on a wiring layer to berecovered. FIGS. 4A and 4B provide examples that illustrate thisadvantage.

Specifically, FIG. 4A illustrates a wiring layer 400 having a diagonalglobal preferred direction. This wiring layer includes a column of powervia arrays 405, an IP Block 410, a set of memory cells 415, and two pins420 and 425. FIG. 4A also illustrates dotted lines that representexamples of diagonal wiring on the layer. The power stripe 405, IP Block410, and the set of memory cells 415 are all obstacles to wiring on thewiring layer. For instance, as shown in FIG. 4A, a problem arises whenpins 420 and 425 need to be connected to each other or to other pins, asthe diagonal wiring that connects to pin 420 is obstructed by the powervia arrays 405 while the diagonal wiring that connects to pin 425 isobstructed by the IP block 410.

In order to solve these routing problems, some embodiments define LPDRsabout these obstacles with the LPDs of these regions different than theglobal preferred direction of the layer. FIG. 4B illustrates examples ofsuch LPDRs. Specifically, this figure illustrates LPDRs 440 that aredefined between the power via arrays and that have a horizontal (0°)LPD. Instead of defining an LPDR between each pair of the adjacent powervia arrays, some embodiments define just one LPDR (with a horizontalLPD) that covers all the aligned power via arrays. This LPDR will havecertain regions (i.e., the regions where the power via arrays exist)blocked for routing. These embodiments define only one LPDR in order tooptimize the runtime processing of the LPDRs, as each LPDR takes upmemory and computational resources.

FIG. 4B also illustrates LPD regions 435 that are between the RAM blocks415 and that have a vertical (90°) LPD. In addition, an LPD region 430having a vertical (90°) local preferred direction is defined between theright side of the IP Block 410 and the right boundary of the wiringlayer, while an LPD region 445 having a horizontal (0°) local preferreddirection is defined between the top side of the IP Block 410 and thetop boundary of the wiring layer. The boundary between LPDR 430 and LPDR445 is defined as a 45° diagonal line in order to increase the capacityof the wiring between two such regions. Defining such boundaries and theadvantages of such boundaries will be further described below.

The LPD regions illustrated in FIG. 4B allow wiring that was previouslyobstructed to now traverse around the obstacles by routing through theseLPD regions along their LPDs. For instance, as shown in FIG. 4B, pins420 and 425 can now be connected through a set of interconnect linesthat traverse along the global 45° direction, traverse through the LPDR440 in the horizontal direction, traverse again along the global 45°direction, and then traverse through the LPDR 445 in the horizontaldirection.

C. Joining Routes At LPD Borders

A common issue to address in LPD routing is how to join together routesegments that traverse two different regions with two different LPDs onthe same layer. Some embodiments of the invention join route segmentstogether along a region that is neither parallel nor perpendicular toeither route segment. FIG. 5 illustrates an example of such a joiningmodel of some embodiments. In this figure, horizontal tracks 505 arelocated in a region with a horizontal local preferred direction, whilevertical tracks 510 are located in a region with a vertical localpreferred direction. As shown in FIG. 5, horizontal and vertical tracksare joined together along a diagonal region 520, which is a diagonalline in this example.

FIG. 6 illustrates another example of such a joining model. In thisfigure, +45° tracks 605 are located in a region with a +45° localdirection, while −45° tracks 610 are located in a region with a −45°local direction. As shown in FIG. 6, 45° and −45° tracks are joinedtogether along a vertical region 620, which is a vertical line in theexample. Some embodiments manifest such joining models in terms ofboundaries between the regions, as further described below in SectionII.

D. Pitch

Some embodiments allow each LPDR to have its own set of pitch values.Other embodiments define a different pitch for each possible routingdirection of each wiring layer. For instance, some embodiments define atleast four (4) pitch values for each wiring layer, with one pitch valuefor each standard direction (horizontal, vertical, 45°, and 135°). Insome embodiments, the distance between each track in each LPD region isset according to the pitch value corresponding to the routing directionin that region.

In some embodiments, the distance between any two parallel tracks is aninteger multiple of the pitch, even when the two parallel tracks are indifferent LPDRs. Offset is the coordinate at the center of a routingtrack (e.g., it is the x-coordinate for vertical tracks). The offset ofthe tracks can be defined for each LPD or can remain undefined to belater determined by the detailed router. In either case, the trackoffsets are defined globally for each wiring direction on a layer. Thus,in some embodiments, there is a common offset for all parallel trackswithin all LPD regions within a particular layer.

Pitches and offsets may vary from layer to layer. If the pitch for aparticular region is left undefined, its value is estimated byapplications (e.g., global and detailed routers). Some embodimentsperform this estimation based on common utility functions that aredependent on minimum size net class and width/spacing of this net class.For example, if just one pitch is defined for a Manhattan direction X,the pitch for the other Manhattan direction Y is automatically estimatedwith the same value by all applications. The same applies for bothdiagonal directions. This means that if just the X-pitch is defined, theY-pitch is defaulted to the same value as X-pitch. In some embodiments,the two diagonal pitches are derived from technology design rules, e.g.,from the minimum spacing and width of the typical nets.

II. LPD Region Generation

Some embodiments of the invention include an LPDR generator thatdesignates regions on one or more layers as LPD regions. In someembodiments, the LPDR generator automatically detects LPDR candidatesand designates some of these candidates as LPD regions. In some of theseembodiments, the LPDR generator also provides the designer with agraphical user interface that allows the designer to specify LPD regionsand to modify the attributes (e.g., boundaries and LPDs) of theseregions. In other embodiments, the LPDR generator does not perform anyautomatic detection and designation of LPDRs, but instead only providesthe designer with a GUI that allows the designer to specify and modifyLPDRs. Also, some embodiments allow a user to define and manipulateLPDRs and LPDs through text-based interfaces, such as a PythonInterface.

The auto-detection and generation of LPDRs is first described below.This discussion is then followed by a discussion of the GUI of the LPDRgenerator of some embodiments of the invention.

A. Auto LPDR Generator

The auto-LPDR generator is intended to make use of LPDRs to increase therouting resources without forcing the user to understand and createLPDRs. As discussed above, LPD creation targets regions the lack routingresources along the global preferred wiring direction of the layer. Suchregions typically exist in the alleys between closely placed macrosand/or between a macro and the layout's boundary. Also, such regions canbe defined between power via arrays used to distribute power in thelayout. Sub-section 1 below first describes defining LPDRs based onmacros, and then sub-section 2 describes defining LPDRs between powervia arrays. It should be noted that some embodiments first define LPDRsbetween the power via arrays, and then define LPDRs based on macros.Alternatively, some embodiment define these LPDRs together.

1. Macros

FIGS. 7 and 8 illustrate examples of regions that are between macros orbetween macros and the layout boundary and that would benefit from LPDwiring. Specifically, FIG. 7 illustrates two macro blocks 715 and 720 ona layer with a horizontal global preferred direction. As shown in thisfigure, the region 705 between the two macros and the region 710 betweenthe macro 715 and the layer boundary provide small amounts of routingspace, which are not particularly useful given the horizontal globalpreferred direction of the wiring on the layer. FIG. 8 illustrates aregion 805 between a macro 810 and the boundary of a layer with a 45°diagonal wiring. Like the regions 705 and 710 of FIG. 7, the regions 805provides a small amount of routing space that is not particularly usefulgiven the 45° global preferred direction of the wiring on the layer.Hence, to make use of regions 705, 710, and 805, the preferred wiringdirections of these regions should be specified differently from theglobal preferred wiring direction of their layer.

FIG. 9 illustrates an auto-LPDR generation process 900 that is used bysome embodiments to generate LPDRs in a layout. This process: (1)identifies candidate regions, (2) designates some or all of thecandidate regions as LPDRs, (3) adjusts the LPDRs for pin access, and(4) modifies LPDRs to improve routability between LPDRs and betweenLPDRs and non-LPDR regions on a layer. This process is described interms of several examples that relate to LPDR generation on layers withManhattan global preferred directions.

As shown in FIG. 9, the process 900 starts by selecting (at 905) alayout layer. The process then decomposes (at 910) the layout layer intoseveral regions. In some embodiments, the process decomposes theselected layout layer by projecting rays from the corner vertices of theoutline of the macro blocks on the selected layer. The outlines of themacro blocks might have been defined prior to 910 or they might bedefined at 910 based on the shape and structure of the content of themacro blocks.

The projected rays are in the direction of the global preferred wiringdirection of the selected layer. FIG. 10 illustrates an example of adecomposition operation on a layer with a horizontal global preferreddirection. In this example, the layer has six macros 1005. Rays areprojected from the vertices of these six macros in the horizontal globalpreferred direction. These projections define thirteen candidate LPDregions 1010.

After 910, the process then selects (at 915) one of the contiguousregions created through the decomposition. It then determines (at 920)whether it should designate the selected region as an LPDR for aparticular local preferred direction. In some embodiment, the processmakes this determination by applying a set of geometric criteria. Thecriteria are meant to ensure that the designation of the selected regionas an LPDR does not remove routing resources from a layer, that usableresources are created by the local preferred direction of the LPDR, andthat sufficient additional resources get created in order to justify theextra runtime and/or memory cost of including an LPDR.

In some embodiments, the criteria for defining LPDRs on a layer with ahorizontal global preferred direction is:

-   -   W_(Max)≧Width of Region≧W_(Min), and    -   Length of Region≧L_(Min),        where the width of the region is in the horizontal direction,        the length of the region is in the vertical direction, W_(Max)        and W_(Min) are upper and lower limits on the width of the        region, and L_(Min) is a lower limit on the length of the        region. The lower limit on region width ensures that the amount        of additional vertical or diagonal resource is worth the cost of        LPDRs. The upper limit on region width ensures that significant        horizontal resources are not lost. Finally, the lower limit on        the region's length ensures that the region is not too short, as        vertical or diagonal tracks that are smaller than some length        might not be of any significant use. This way the additional        vertical or diagonal routes will have significant movement along        the vertical or diagonal direction.

In some embodiments, the criteria for defining LPDRs on a layer with avertical global preferred direction is:

-   -   L_(Max)≧Length of Region≧L_(Min), and    -   Width of Region≧W_(Min).        Again, the width of the region is in the horizontal direction,        the length of the region is in the vertical direction, L_(Max)        and L_(Min) are upper and lower limits on the length of the        region, and W_(Min) is a lower limit on the width of the region.        The lower limit on region length ensures that the amount of        additional horizontal or diagonal resource is worth the cost of        LPDRs. The upper limit on region length ensures that significant        vertical resources do not get lost. Finally, the lower limit on        the region's width ensures that the region is not too thin, as        horizontal or diagonal tracks that are smaller than some length        might not be of any significant use. This way the additional        horizontal or diagonal routes will have significant movement        along the horizontal or diagonal direction. Some embodiments        define the criteria for defining LPDRs on a layer with diagonal        global preferred directions similarly.

If the process determines (at 920) that the selected region is not agood candidate for an LPDR, the process transitions to 930, which willbe described below. On the other hand, when the process determines (at920) that the selected region is a good LPDR candidate, it transitionsto 925, where it designates the selected region as an LPDR. At 925, theprocess also designates the LPD of the selected region.

For a layer that has a Manhattan global preferred direction, someembodiments define the LPD of a designated LPDR on that layer as theManhattan direction that is orthogonal to the layer's Manhattan globalpreferred direction. On a layer that has a diagonal global preferreddirection, some embodiments define the LPD of a designated LPDR on thatlayer as one of the Manhattan directions. This Manhattan direction mightbe a direction that is identified by the dimensional attributes (e.g.,orientation) of the LPDR. For instance, when the LPDR is a tall andnarrow rectangle aligned with the y-axis, the LPD direction might bedesignated as the vertical direction. Alternatively, some embodimentsdefine the LPD of an LPDR on any layer based on the dimensionalattributes of the LPDR. Also, some embodiments define the LPD of an LPDRthat is defined between two or more macro blocks based on the positionalrelationship of the macro blocks. For instance, FIG. 11 illustrates a45° LPD for an LPDR 1105 that is defined between two macro blocks 1110and 1115 that are diagonally offset from each other, because itfacilitated routing between the open areas 1120 and 1125 to reduce theimpact of the macroblocks.

FIG. 12 illustrates the elimination of several candidate LPD regions,which were created in the tessellation illustrated in FIG. 10, forfailing to satisfy the above-described width and length criteria. Inthis example, the remaining LPDRs (i.e., the LPDRs illustrated in thisfigure) all have been assigned a vertical LPD.

After 925, the process transitions to 930. At 930, the processdetermines whether it has examined all the contiguous regions created onthe selected layer by the decomposition operation at 910. If not, theprocess selects (at 915) another contiguous region, determines (at 920)whether this region should be designated as an LPDR, and (3) in case ofan affirmative determination at 920, designates (at 925) the selectedregion as an LPDR.

When the process determines (at 930) that it has examined all thecontiguous regions created on the selected layer by the decompositionoperation at 910, the process examines (at 935) each particular regionthat it designated (at 925) as an LPD region to determine whether itneeds to adjust or eliminate this region based on pins at the boundariesof the particular region.

The newly created LPDRs should not hinder pin access. Hence, the process900 needs to ensure that the LPDRs provide a safe distance for access tothe pins. Accordingly, for each particular LPDR defined at 925, theprocess initially determines (at 935) whether there is at least one pinon one side of the particular LPDR that needs to connect to another pinon another side of the LPDR. If so, the process discards the LPDR insome embodiments, as the LPDR would block the easiest way to connect thetwo pins. FIG. 13 presents an example of an LPDR 1305 that is created ona layer between two macros 1310 and 1315. On the two sides of the LPDR1305, the two macros have two pins 1320 and 1325 that need to connect.Hence, as shown in FIG. 13, the pin adjustment operation at 935 removesthe LPDR 1305 from the layout. Instead of discarding (at 935) an LPDR,the process 900 in some embodiments tries to modify (at 935) the shapeof the LPDR (e.g., tries to make the LPDR narrower or shorter as furtherdescribed below) when it determines that there is one pin on one side ofthe particular LPDR that needs to connect to another pin on another sideof the LPDR. If the modification fails to lead to an acceptablesolution, the process then discards the LPDR in some embodiments.

The process 900 also identifies (at 935) each LPDR defined at 925 thathas one or more pins on its sides even when the pins do not need toconnect across the LPDR. For each such LPDR, the process (1) changes theshape of the LPDR to create one or more open corridors for pin access,and (2) then determines whether the modified LPDR still satisfies theabove-mentioned criteria for creating the LPDR. If the modified LPDR nolonger satisfies one or more of the criteria (e.g., the modified LPDR'swidth is smaller than the required minimum width), the process discardsthe LPDR. Otherwise, the process keeps the LPDR with its modified shape.

FIG. 14 illustrates an example of a pin adjustment operation thatmodifies the shape of an LPDR. Specifically, this figure presents anexample of an LPDR 1405 that is created on a layer between two macros1410 and 1415. On one side of the LPDR 1405, the macro 1410 has a pin1420 that needs to be connected. Hence, as shown in FIG. 14, the pinadjustment operation at 935 makes the LPDR 1405 narrower (i.e., reducesits width).

Modifying the shape of an LPDR provides sufficient routing flexibilityfor accessing the pins. This leeway can be used by routes to either viaout of the layer (the way they would have done without LPDRs) or to joginto the LPDR and blend into the flow. For the LPDR modificationillustrated in FIG. 14, FIG. 15 illustrates an example that shows theuse of the corridor 1405 created by the adjustment for a via access tothe pin. For this modification, FIG. 16 illustrates an example of howsome embodiments allow access to the pin through a 45° jog 1610 into themodified LPDR 1405, a vertical traversal through this LPDR, and thentraversing back in the 45° direction after leaving the LPDR. Given thatjog 1610 is not along a the LPD of the region 1405, some embodimentsassess this jog a penalty for traversing a portion of this region alongits non-preferred direction.

The amount that a dimension of the LPDR is adjusted is dependent on thenumber of pins on the side or sides of the LPDR that are associated withthat dimension. For instance, some embodiments deduct the followingdistance D from each side of an LPDR region:D=Max (S _(Min) , P _(Max)*pitch*C _(Pin))where P_(Max) is the number of pins along the edges of the LPDR, pitchis the wiring pitch along the global preferred direction, C_(Pin) is apin routing cost that is a heuristic parameter that quantifies the costof a number of tracks that have to be left aside per pin, and S_(Min) isthe minimum spacing requirement for pin access, which is defined by thedesign rules. Some embodiments drop the min-spacing requirement from theabove formula in order to simplify it as follows:D=P _(Max)*pitch*C_(Pin)

Some embodiments might use different rules for performing pinadjustments on layers with diagonal global preferred directions than onlayers with Manhattan global preferred directions. For instance, someembodiments may choose to maintain the diagonal direction in avertically/horizontally shaped region and hence may discard an LPDR whenthere is a pin at the boundary of the LPDR.

FIG. 17 illustrates another example of the pin adjustment operation.This example is a continuation of the examples illustrated in FIGS. 10and 12. The top layout illustration in FIG. 17 presents several pins onthe sides of the macros 1005. Next, the bottom left layout illustrationin FIG. 17 shows the elimination of LPDR 1010. This LPDR was eliminatedbecause two pins 1705 and 1710 on its sides need to be connected to eachother. The bottom left layout also illustrates the narrowing of LPDRs1015 and 1020 to create corridors for accessing pins on the side ofthese two LPDRs. Finally, the bottom right layout in FIG. 17 illustratesthe elimination of the narrowed LPDR 1015 for failing to satisfy theminimum width criteria for a vertical LPDR.

After performing the pin adjustment operation at 935, the processperforms a boundary adjustment operation at 940. In some embodiments,the routability between two regions is dependent on their routingdirections and the orientation of the edge separating the two regions.Specifically, when one of the wiring directions between two regions on alayer is parallel to a boundary between the two regions, then someembodiments define the capacity at the boundary between two regions aszero. Such a boundary is referred to as an impermeable boundary betweenthe two wiring directions.

To avoid such impermeable boundaries, the process 900 performs theboundary adjustment operation at 940 that changes the boundary betweentwo regions to eliminate any impermeable boundaries between them. FIGS.18-20 illustrate examples of impermeable boundaries between regions on alayer and examples of eliminating such boundaries by reshaping theregions. Specifically, FIG. 18 illustrates a layer with a horizontalglobal preferred wiring direction and a LPDR 1805 between two macros1810 and 1815. As shown in this figure, the LPDR 1805 has a verticalLPD. The LPDR 1805 also has horizontal top and bottom sides 1820 and1825 that are parallel to the horizontal global preferred wiringdirection of the layer. Accordingly, some embodiments define thewireflow capacity across the top and bottom sides 1820 and 1825 as zero(i.e., define these sides as impermeable sides). Such a wireflowdefinition is a conservative definition as a detailed router might allownon-preferred direction jogs at such boundaries.

The permeability at the boundary of the LPDR 1805 and the region withthe global preferred wiring direction can be improved by modifying theshape of this boundary. For instance, FIG. 19 illustrates the additionof triangular crown regions 1905 and 1910 to the top and bottom sides ofthe LPDR 1805. Each triangular crown extension of the LPDR 1805 includesa 45° edge and a vertical edge. In some embodiments, the 45° edge is apermeable edge for routes to enter and exist the LPDR, while thevertical edge is an impermeable edge as it is parallel to the verticalLPD of the LPDR 1805. FIG. 19 illustrates an example of a route 1920that traverses through the LPDR 1805 through its permeable edges.

FIG. 20 illustrates the additions of alternative triangular crownregions 2005 and 2010 to the top and bottom sides of the LPDR 1805. Eachof these triangular crown extensions of the LPDR 1805 includes two 45°edges, both of which are in some embodiments, permeable edges. FIG. 21illustrates two routes 2105 and 2110 that enter the LPDR 1805 throughthese permeable edges. However, as shown in FIG. 21, the route 2105includes a non-preferred direction jog in the LPDR 1805. Hence, theadded advantage of the two extra permeable edges that are provided bythe crown extensions 2005 and 2010 come at the expense of requiring someroutes to have non-preferred direction jogs in the LPDR.

LPDR crown extensions provide well-defined bending points for theroutes. These well-defined points are only as strict as the layerdirection itself. Accordingly, in some embodiments, the same kind ofjogs that can run orthogonal to a routing direction can also violate thebending points in case the benefit offsets a higher price of anon-preferred-direction jog.

Some embodiments use the following approach to modify an impermeableboundary of an LPDR on a Manhattan layer. The impermeable boundary ofthe LPDR abuts two edges of the LPDR that abut the boundary edges of thelayer or of macros on the layer. Each of these two edges is checked todetermine whether it can be extended. This involves checking the boundsof the macro's edge next to it. The end-point of the edge is termedextendible if the edge can be elongated at that end-point withoutextending beyond the macro's edge. The amount, by which the edge needsto extend for stretching up to the obstruction edge, is the ExtendLimit.The extendibility and the ExtendLimit are determined at four points,which are two endpoints of both the edges.

The following description provides an example of the stretching logicthat is performed to stretch a vertically shaped LPDR. When the LPDR canbe extended at both its left top corner and right top corner, then theprocess discards the LPDR as there was some error in its creation.Alternatively, when the LPDR can be extended at its left top corner butnot its right top corner, then the process stretches the LPDR's leftedge upwards by the minimum of the LPDR width and a maximum top stretchlimit. If the LPDR's left edge cannot be stretched by this minimumamount, then the LPDR is discarded in some embodiments.

When the LPDR can be extended at its right top corner but not its lefttop corner, then the process stretches the LPDR's right edge upwards bythe minimum of the LPDR width or a maximum top stretch limit. If theLPDR's right edge cannot be stretched by this minimum amount, then theLPDR is discarded in some embodiments. When the left and right topcorners of the LPDR cannot be extended, then the process connects theleft edge and the right edge by a 45° edge and a 135° edge respectively.The 45° and 135° edges should not exceed the top stretch limit. Iftruncated, a horizontal line should connect the 45° and 135° edges.Stretching the bottom boundary of a vertical LPDR or the right and leftsides of a horizontal LPDR follows an analogous set of operations forthe bottom, right, and left sides of an LPDR.

FIG. 22 illustrates another example of the boundary adjustmentoperation. This example is a continuation of the examples illustrated inFIGS. 10, 12, and 17. This figure illustrates the creation of crownextensions for each of the LPDRs with vertical LPDs. Crown extensions2205, 2210, and 2215 are triangular extensions extended from one side ofthe LPDR, while crown extensions 2220, 2225, and 2230 are triangularextensions extended from both sides of the LPDR. Extensions 2235 and2240 are four sided extensions that resulted because of the boundary ofthe layer or because of the minimum spacing requirement for pin access.

Some embodiments might use different rules for performing boundaryadjustments on layers with diagonal global preferred directions than onlayers with Manhattan global preferred directions. An LPDR, with aManhattan LPD and a Manhattan outline on a layer with diagonal globalpreferred wiring, is always permeable itself. However, the LPDRs createdaround it can deteriorate its permeability/routability. Accordingly, foreach macro on a diagonal layer that has LPDRs on two consecutive sides,some embodiments extend the two LPDRs to join them at the corner vertexwhere the sides meet. The modus operandi of this extension is to extendthe Manhattan bound of region-end-point to a large value, and constrainthe region with a diagonal bound. The bound is stretched diagonallyoutward from the vertex of the two consecutive sides.

FIG. 23 illustrates an example of such an extension. Specifically, thisfigure illustrates expanding two LPDRs 2305 and 2310 that abut a macro2315 to improve the routability between these LPDRs and the rest of thelayer. Such a solution might lead to odd boundaries between the LPDRs,such as the contact between LPDRs 2405 and 2410 that are illustrated inFIG. 24. This contact creates an impermeable edge 2415 between the LPDR2410 and the rest of the layer as it is parallel to the global preferredwiring direction of the layer. Such an impermeable edge is createdbecause the height of LPDR 2405 is smaller than the width of LPDR 2410.In such cases, the two LPDRs might not be extended at all, might beextended as shown in FIG. 24 but then corrected during a manual LPDRcreation by a designer, or might be extended in a manner that results inthe pentagonal shape for LPDR 2510 that is illustrated in FIG. 25.

In extending LPDRs, the boundary adjustment operation at 940 might leadto the LPDRs overlapping other LPDRs. So, after the boundary adjustingoperation at 940, the process 900 checks (at 945) all LPDRs on theselected layer to make sure that no two LPDRs overlap. When itidentifies two LPDRs that overlap, it deletes (at 945) one of them(e.g., the smaller LPDR) in the region of the overlaps. After 945, theprocess determines (at 950) whether it has examined all the wiringlayers. If not, the process returns to 905 to select another wiringlayer and then performs the subsequent operations to potentially defineone or more LPDs on this layer. When the process determines (at 950)that it has examined all the wiring layers that it needs to examine, theprocess ends.

2. Power Via Arrays

Power structures often reduce the routing resources that are availableon the wiring layers. Power via arrays are one example of such powerstructures. A power via array includes a set of vias that are used toroute power from power lines (also called power stripes) on the topmostmetal layers down into the lower metal layers. These power stripesrequire Manhattan directed wiring to access the set of vias in the powerarray. Accordingly, as discussed above, problems in routing arise whentrying to route wiring on a diagonal layer with Manhattan power stripes.As further described above, some embodiments solve this problem bydefining LPDRs with Manhattan LPDs for horizontally or verticallyaligned power via arrays.

FIG. 26 illustrates a process 2600 that the LPDR generator of someembodiments uses to create LPDRs between power via arrays on diagonalwiring layers. This process creates LPDRs starting from the top-mostdiagonal layer and moves down until it finishes with all the diagonallayers. In a layer, the process considers all the power stripes in someembodiments, while considering only power stripes greater than aconfigurable threshold size (e.g., 100 micron) in other embodiments. Theprocess ignores all diagonal power stripes. For each Manhattan powerstripe that is greater than the threshold size, the process creates anLPDR with the same outline as the stripe. The LPD of the LPDR will behorizontal for a vertically shaped LPDR, and vertical for a horizontallyshaped LPDR. For each potential LPDR, the capacity will be calculatedfor the Gcells containing the LPDR (with and without the LPDR). If theLPDR increases the capacity at least two-fold, the LPDR is added to thedatabase. Otherwise, the LPDR will be discarded.

As shown in FIG. 26, the process 2600 identifies (at 2605) the number ofrouting layers and the diagonal layers with no power stripes. Theprocess 2600 then iterates (at 2610) through the power and ground netsin the netclass database to identify each Manhattan power stripe that ison a layer above the lowest diagonal layer and that is larger than aparticular configurable threshold size. In some embodiments, thethreshold size of the Manhattan power stripe is 100 microns, althoughthis size can be redefined by a designer. Each Manhattan power stripeidentified at 2610 might be used to define an LPDR on one or morediagonal wiring layers below it, as further described below. To identifypower vias, some embodiments might represent an entire power via-stackthat includes multiple cuts as one via.

Next, at 2615, the process defines the Current-Layer as the topmostdiagonal layer. It then selects (at 2620) the nearest layer that isabove the Current-Layer and that has Manhattan power stripes. Theprocess then determines (at 2625) whether the selected layer above theCurrent_Layer has a sufficient number of (e.g., ten) Manhattan powerstripes. The number of power stripes that are sufficient is configurablein some embodiments.

If the process determines that the selected layer does not have asufficient number of Manhattan power stripes, the process transitions to2640, which will be described below. Otherwise, the process evaluates(at 2630) the outline of each particular Manhattan stripe as a potentialLPDR on the Current_Layer.

Specifically, for each potential LPDR that can be defined based on eachparticular Manhattan stripe, the process performs (at 2630) two capacitycomputations for the set of Gcells that contain the potential LPDR. Onecapacity computation is the total capacity of all the Gcells in the setwithout the potential LPDR, while the other one is the total capacity ofthese Gcells with the potential LPDR. The capacity calculation isperformed with power/ground vias taken as obstructions. When thepotential LPDR fails to increase the capacity of the set of Gcells atleast two-fold, the process does not define an LPDR. Alternatively, onthe Current_Layer, the process defines (at 2630) an LPDR based on theoutline of the particular Manhattan power stripe when the potential LPDRincreases the capacity of the set of Gcells at least two-fold.

FIG. 27 illustrates an example of creating LPDRs about power structures.Specifically, this figure illustrates a wiring layer that has a diagonalglobal preferred direction. This layer also has a region 2705 that isunderneath a vertical power stripe. Hence, as shown in FIG. 27, theregion 2705 can be defined as an LPDR that has a horizontal LPD. ThisLPD, in turn, provides horizontal routing and/or tracks that allowsdiagonal routing and/or tracks to pass through this region whileavoiding the power-via obstacles that are defined for the vertical powerstripe.

After 2630, the process 2600 examines (at 2635) any LPDRs that it justcreated at 2630 to determine whether to merge adjacent LPDRs. DefiningLPDRs for adjacent power stripes might create unusable channels betweenthe LPDRs. FIG. 28 illustrates two LPDRs 2810 and 2820 that are definedfor two different power stripes. In this example, the LPD in each LPDRis horizontal and the global preferred wiring direction of the layer is45° diagonal. As shown in this figure, the wiring that leaves the LPDR2810 that is defined for one power stripe can run into a power via stack2815 in the LPDR 2820 of the other power strip.

FIG. 28 illustrates the merging of the two LPDRs 2810 and 2820 to definea new LPDR 2830. The “merged” LPD region 2830 is defined to encompassthe region of both power stripes and has the same local preferreddirection as the LPD of the replaced LPDRs 2810 and 2820. This mergingallows the wiring to traverse efficiently across the region underneaththe power stripes without the obstruction that existed when the twoLPDRs 2810 and 2820 were separate.

Some embodiment merge power-stripe LPDRs that are closer than 10% of thestripe-width. Some embodiments perform the merging after thecapacity-increase-based LPDR filtering because they assume that a regionthat does not gain capacity from a change to its routing direction, willnot gain capacity even if merged with another LPDR. Other embodiments,however, might account for the merging while performing the capacityestimation and determining whether to define an LPDR.

In some embodiments, the merging operation merges two aligned (e.g.,horizontally aligned) LPDRs by extending one LPDR (e.g., the LPDR to theleft) towards the other LPDR (e.g., the LPDR to the right). The formulabelow quantifies the horizontal extension (HExt) of leftside LPDRtowards a rightside LPDR:HExt=(HSeparationOfStripe−viaStackOffset) mod interval,where HSeperationOfStripe is the horizontal separation of the powerstripes, viaStackOffset is the amount of offset between the via stack inthe left stripe and the via stack of the right stripe, and interval isthe distance between the two via stacks that are part of the stripecorresponding to the left LPDR. Some embodiments put a ceiling on theextension to make sure that not more than a particular percent of thelayer is converted to LPDRs. For example, a Horizontal extension willtake place only if the value is less than the stripe's width. A valuegreater than the ceiling is ignored, as it would not be helpful to drawanything less than the horizontal value. Hence, in such cases, someembodiments do not define the LPDR that would need to surpass theceiling.

A second step after the extensions would be to check whether an LPDR hasextended into the next LPDR. If so, the merging operation merges thedefinition of the two LPDRs, provided that they pass a capacityconstraint, which will be described below. The above-described approachassumes that the via-stacks within a stripe are placed at regularintervals, that the interval remains the same for the two stripes beingconsidered, and that power-vias do not lie outside the stripes. Someembodiments incorporate a check for such requirements at the beginningof the merging operation.

As mentioned above, the merging operation at 2635 performs anothercapacity-increase-based filtering. Unaligned vias can cause the failureof capacity increase. In case of a failure, the process in someembodiments discards the merged LPDR. Instead of performing apost-processing operation to merge LPDRs for adjacent power stripes,some embodiments might generate larger LPDRs at 2630 that account forthe need to have a combined LPDR for adjacent power stripes.

After 2635, the process determines (at 2640) whether there is anydiagonal layer lower than the Current_Layer. If so, the process selects(at 2645) the next lower layer, designates this layer as theCurrent_Layer, and transitions back to 2620, which was described above.Otherwise, the process ends.

Some embodiments define LPDRs about power structures in view of certainconstraints. For instance, some embodiments do not create LPDRs aroundoverlapping power stripes. In some cases, a designer has to manuallyanalyze the LPDRs to ensure that this constraint is met. Also, asmentioned above, the LPDR generator in some embodiments first definesLPDRs about power structures and then defines LPDRs between the macros.Accordingly, the LPDR generator in some embodiments does not checkwhether the power-based LPDRs overlap any other existing LPDRs. In fact,the LPDR generator might delete all pre-existing LPDRs before creatingany power-based LPDRs on a layer.

FIG. 29 illustrates an example of an alternative embodiment of theinvention. This figure illustrates the outlines 2905 of several powerstripes on a layer. It also illustrates several LPDRs that areorthogonal to the outline of the power stripes, instead of being definedparallel and in between the stripes.

B. Manual LPDR Generation

U.S. patent application entitled “Local Preferred DirectionArchitecture, Tool, and Apparatus” filed concurrently with the presentapplication, with the Express Mail Number EV454047165US, and attorneydocket number CDN.P0076, describes the GUI of the LPDR generator of someembodiments of the invention. This application is incorporated herein byreference.

III. Global Routing

Routing is at times performed in two stages, a global routing stage anda detailed routing stage. Global routing provides a general routing planfor nets in a layout. Detailed routing provides the specific routingplan for nets in a layout.

In the embodiments described below, the router partitions an IC-layoutregion into several square sub-regions. For each net being routed, therouter then identifies a global route that connects the set ofsub-regions that contain at least one pin of the net. Each net's globalroute is a set of edges (i.e., interconnect lines) that connects the setof sub-regions that contain the net's pins. The identified routes mighthave horizontal, vertical, and ±45° diagonal edges in the embodimentsdescribed below.

These edges are defined within a routing graph (also called a Groutegraph) that is first described below. The overall flow of the router isthen described, followed by a discussion of the computation of thecapacity of edges in the routing graph in view of the local preferreddirections of the layout.

A. Routing Graph

In some embodiments, the router uses two grids to create a Groute graph.The first grid is a coarser grid that divides the IC layout into anumber of sub-regions, called Gcells. The second grid is a finer gridthat divides each Gcell into four sub-regions. In the embodimentsdescribed below, the Gcells are square. This shape supports ±45°routing, as any set of ±45° wiring tracks that cut through a squareGcell will fill its horizontal and vertical boundaries consistently. Oneof ordinary skill will realize that other embodiments might usedifferent shaped Gcells.

On each wiring layer, each of the four sub-regions in each Gcell isrepresented by a node at the center of the sub-region. The embodimentsdescribed below use the coarser grid to measure route congestion in thelayout region, and use the finer grid to measure route lengths.Accordingly, below, the coarser grid is referred to as the congestiongrid, while the finer grid is referred to as the length grid.

FIGS. 30 and 31 illustrate small sections of the congestion and lengthgrids. As shown in these figures, intersecting horizontal and verticallines form both these grids. FIG. 30 illustrates a 4×4 section of thecongestion grid 3000. This section divides a portion of an IC regioninto 16 Gcells 3005. In the embodiments described below, the congestiongrid divides the IC region into many more Gcells (e.g., tens or hundredsof thousands).

FIG. 31 illustrates a section of the length grid 3100 that correspondsto the section of the congestion grid 3000 illustrated in FIG. 30. Asshown in this figure, the length grid divides each Gcell 3005 into fournodes 3105 on each wiring layer. FIG. 32 illustrates the four nodes ineach Gcell on a particular layer. There are a number of planar andnon-planar edges between the nodes defined by the length grid 3100.These edges are referred to as “node edges” in the discussion below.

A planar node edge connects two adjacent routing-graph nodes. Each suchedge represents a set of wiring tracks along the edge's particulardirection that connect the two sub-regions represented by the edge's twonodes. Planar node edges have different directions on different wiringlayers.

Several examples of planar edges are illustrated in FIGS. 33-41. Thefirst set of these examples, which are illustrated in FIGS. 33-36,ignore possible local preferred directions on the wiring layers, inorder to simply convey the notion of the planar edge on a layer. Thenext set of examples, which are presented in FIGS. 37-41, then provideillustrations of planar edges on layers with more than one preferreddirection (e.g., a layer with a global preferred direction and one ormore local preferred directions).

Ignoring possible local preferred directions, FIGS. 33-36 illustrateplanar edges on layers 2-5 along the global preferred directions ofthese layers in some embodiments. Some embodiments assume that there areno planar node edges between routing-graph nodes on layer 1, as thislayer is often quite congested. Some of these embodiments promote allthe pins on layer 1 to layer 2. Other embodiments, however, specifyplanar node edges on layer 1. In some of these embodiments, the planarnode edges on layer 1 are in the same direction as planar node edges onlayer 3.

FIG. 33 illustrates that on layer 2 a vertical node edge 3305 existsbetween each pair of vertically adjacent nodes in a region on layer 2that is to be routed according to the vertical global preferreddirection of layer 2. FIG. 34 illustrates that on layer 3 a horizontalnode edge 3405 exists between each pair of horizontally adjacent nodesin a region on layer 3 that is to be routed according to the horizontalglobal preferred direction of layer 3.

FIG. 35 illustrates that 45° diagonal node edges exist between northwestnodes 3505 and southeast nodes 3510 of different Gcells in a region onlayer 4 that is to be routed according to the 45° diagonal globalpreferred direction of layer 4. As shown in this figure, no 45° diagonalnode edges are incident on northeast nodes 3515 and southwest nodes3520. FIG. 36 illustrates that −45° diagonal node edges exist betweennortheast node 3515 and southwest nodes 3520 of different Gcells in aregion on layer 5 that is to be routed according to the −45° globalpreferred direction of layer 5. As shown in this figure, no −45°diagonal node edges are incident on northwest nodes 3505 and southeastnodes 3510.

For embodiments that allow local preferred directions on layers 2-5,FIGS. 37-40 illustrate potential planar edges along potential localpreferred directions on layers 2-5. These edges are potential edges asthey would only be defined if the nodes that they connect fall withinregions that have their local preferred directions coincide with thedirection of the edges.

For instance, for layer 2, FIG. 37 illustrates a potential horizontalnode edge 3705 between each pair of horizontally adjacent nodes and apotential −45° edge 3710 between northeast and southwest nodes ofdifferent Gcells on layer 2. The horizontal edges 3705 would only bedefined when the nodes that they connect fall within regions on layer 2that have a horizontal local preferred direction. Similarly, the −45°edges 3710 are only defined when the nodes that they connect fall withinregions on layer 2 that have a −45° local preferred direction.

For layer 3, FIG. 38 illustrates a potential vertical node edge 3805between each pair of vertically adjacent nodes and a potential 45° edge3810 between northwest and southeast nodes of different Gcells on layer3. For layers 4 and 5, FIGS. 39 and 40 illustrate a potential verticalnode edge 3905 between each pair of vertically adjacent nodes and apotential horizontal node edge 3910 between each pair of horizontallyadjacent nodes on layers 4 and 5.

As shown in FIGS. 37-40, some embodiments allow only one diagonaldirection on any layer. Some embodiments allow different diagonaldirections for planar edges on the same layer, but never on the sameGcell boundary. FIG. 41 illustrates an example of a routing graph forlayer 3 in a case where layer 3 has a horizontal global preferreddirection and two regions 4105 and 4110 that respectively have a −45°and +45° local preferred directions.

This figure illustrates numerous horizontal planar edges 4115 betweenhorizontally adjacent nodes on layer 3 that are not within regions 4105and 4110. It also illustrates numerous horizontal planar edges 4115between horizontally adjacent nodes along the boundaries of the regions4105 and 4110, when the sub-regions represented by these nodes is notentirely within the regions. This figure also illustrates several −45°planar edges 4120 in region 4105, and several 45° planar edges 4125 inregion 4110. The directions of these planar edges coincide with thelocal preferred directions of the two regions 4105 and 4110.

In some embodiments, each Manhattan node edge has a unit length cost(L). In these embodiments, each diagonal node edge has a length costthat equals the unit length cost times the square root of two (L*{squareroot}{square root over (2)}). Also, the use of a node edge across aGcell boundary reduces the capacity of the boundary, and is therebyassessed a wire congestion cost.

The router examines wire congestion at Gcell boundaries on each layeravailable for routing. Specifically, the router computes capacities atGcell boundaries on each routing layer. On a particular layer, thewiring resources (i.e., wiring tracks) across a Gcell boundary can beconceptually represented as a planar “congestion edge” across thatboundary on the particular layer. The capacity of a congestion edgebetween two Gcells is the wiring capacity for all available wiringdirections between the two Gcells.

For a boundary between two Gcells that do not fall in a region with alocal preferred direction that is different than the layer's globalpreferred direction, the allowed wiring direction is the globalpreferred direction. For a boundary between two Gcells that fall withintwo or more regions with different preferred directions, the allowedwiring directions are the two or more wiring directions in someembodiments. FIG. 42 illustrates an example of the planar congestionedges that are used to measure congestion along layer 3 for the exampleillustrated in FIG. 41.

As illustrated in FIGS. 33 and 34, up to two vertical or horizontalplanar edges can cross the boundary between each pair of vertically orhorizontally adjacent Gcells in some embodiments. In addition, asillustrated in FIGS. 37-40, a diagonal edge can cross the boundarybetween two Gcells. Hence, the congestion along two horizontally alignedGcells can be attributable to obstacles or routes along the twohorizontal planar edges and the one diagonal planar edge that cross theboundary between these Gcells. Similarly, the congestion along twovertically aligned Gcells can be attributable to obstacles or routesalong the two vertically planar edges and the one diagonal planar edgethat crosses the boundary between these Gcells.

B. Overall Flow of the Router

FIG. 43 illustrates a process 4300 that conceptually represents theoverall flow of the router in some embodiments of the invention. Asshown in this figure, the process 4300 initially uses (at 4305) thecongestion and length grids 3000 and 3100 to partition the IC layoutregion into numerous Gcells, with four nodes on each routing layer ineach Gcell. As described above, these Gcells and nodes define a Groutegraph in which the router defines and embeds routes. The Groute graphincludes a grid for each wiring layer.

Next, at 4310, the process selects one of the wiring layers. The processthen identifies (at 4315) edges between the nodes of the routing graphfor the selected layer, based on the global preferred wiring directionand the local preferred wiring directions (if any) of the selectedlayer. As described above by reference to FIGS. 37-40, the routing graphfor each layer includes numerous potential horizontal, vertical, anddiagonal edges. Hence, at 4315, the process examines each potential edgein the routing graph of the selected layer. The process specifies anactual edge in the selected layer's routing graph for each potentialedge that matches a certain set of criteria. This identification processwill be further described in Section C below.

At 4315, the process also defines the capacity at each Gcell boundary onthe selected layer based on the global preferred wiring direction andthe local preferred wiring directions (if any) of the selected layer. Asmentioned above, on a particular layer, the wiring resources (i.e.,wiring tracks) across a Gcell boundary can be conceptually representedas a planar “congestion edge” across that boundary on the particularlayer. The capacity of a congestion edge between two Gcells is thewiring capacity for all available wiring directions between the twoGcells.

Different embodiments compute the capacity of edges in the globalrouting graph differently. For instance, some embodiments compute thecapacity of edges in the Groute graph that correspond to regions in thelayout with multiple different preferred directions based on novelcapacity estimation techniques, while computing the capacity of edges inthe Groute graph that correspond to regions in the layout with only onepreferred direction based on existing capacity estimation techniques.Edge capacity computations will be further described in Section D below.

At 4315, the process also computes the via capacity for each Gcell.Different embodiments compute the via capacity differently. In a givenGcell, some embodiments compute the via capacity based on an area-basedsampling of open areas within the Gcell. Other embodiments compute thevia capacity between two adjacent layers in a Gcell as a user-adjustableconstant times the maximum of all capacities of planar congestion edgesinto the Gcell on either of the layers.

After 4315, the process determines (at 4320) whether it has examinedeach wiring layer. If not, the process returns to 43 10 to selectanother layer, and then performs the edge identification and capacitycomputations for this layer 4315. When the process determines that ithas examined each routing layer, it transitions to 4325.

At 4325, the process selects a net for routing. It then specifies (at4330) source and target sets for performing a path search for theselected net, and then performs a path search at 4335. For the firstpath search that the process performs to define a route for the selectednet, the process often selects a node set that is associated with aparticular pin of the net as the target set and specifies the nodes ofthe nearest K pins (where K can be 1) of the net as the source set. Thenodes include any Steiner nodes that might have been defined for thenet. For any additional path search that is performed to define theroute for the selected net, the process defines (1) the target set asall the nodes that are associated with the routed pins and Steinerpoints of the net, and all nodes that are currently on the one or morepaths embedded for the net during the current route generation, and (2)the source set as all nodes associated with any unrouted pin and Steinerpoint in the net's configuration that are within a certain distance ofthe target set.

The path search performed at 4335 is an A* path search that at eachiteration tries to extend a partial solution with the best estimatedcost. Specifically, during its path exploration phase, the processstarts its path search by specifying the start of one or more paths fromone or more source nodes. It then iteratively identifies one or morepath expansions about the lowest cost path, until it identifies a paththat connects a source node and a target node. Each identified expansionabout a path is from a “current node” (also called “start node”) reachedby the path being extended to a “destination node” that neighbors thecurrent node.

During the path search, the process examines the capacity at a Gcellboundary when it identifies an expansion across such a boundary, inorder to ascertain the viability of the expansion. For instance, FIG. 44illustrates a path search that starts at a node 4405 and ends at a node4410. In reaching node 4410, this path search has explored variousexpansions. One of the expansions that has been identified is theexpansion 4415 from node 4420 to node 4410 across Gcell boundary 4425.When the path search operation identifies this expansion, it determineswhether the Gcell boundary 4425 has sufficient available capacity todefine another route across this boundary in the direction of theexpansion 4415. The available capacity across a Gcell boundary (such asboundary 4425) equals the initial capacity that the global routercomputed at 4315 minus the estimated number of tracks that it has sincedefined across that boundary. When the path search operation determinesthat an identified expansion is crossing an overcongested Gcellboundary, it either discards the expansion as a non-viable expansion orassigns this expansion a high cost.

When the path search at 4335 identifies a path between the source andtarget sets identified at 4330, the process embeds (at 4335) theidentified path and then transitions to 4340. At 4340, the processdetermines whether it has defined a complete route for the net selectedat 4325 (i.e., whether it has defined a route that connects all the pinsof the net). If not, the process transitions back to 4330 to define newsource and target sets for another path search, and then performs thispath search to further define the route for the selected net.

When the process determines (at 4340) that it has completely defined theroute for the selected net, it determines (at 4345) whether it hascompleted its routing of all the nets. If not, the process transitionsback to 4325 to select another net for routing. In some instances, theglobal routing process 4300 rips up and redefines routes for aparticular net one or more times, in order to facilitate the routing ofother nets or to improve the routing of the particular net. Also, insome instances, one or more of the path search operations that therouting process performs for a particular net might not identify a pathbetween source and target sets. In these instances, the routing processtries several times to identify such a path, and when it fails, it flagsthe net as one that it was not able to route. When the process 4300determines (at 4345) that it has completed its routing of the nets, itterminates.

C. Edge Identification

As described above by reference to FIGS. 37-40, the routing graph foreach layer includes numerous potential horizontal, vertical, anddiagonal edges. At 4315, the global routing process 4300 specifies eachpotential edge in the routing graph of the selected layer as an actualedge if the potential edge matches a set of criteria.

For instance, in some embodiments, the process specifies an actual edgein the selected layer's routing graph for each potential edge that hasits direction match the global preferred direction or one of the localpreferred directions of the selected layer. A direction of a potentialedge matches a preferred direction on the selected layer when thepotential edge's direction matches the preferred wiring direction of atleast a portion of the sub-region that is represented by a nodeconnected by the potential edge. The above-described FIG. 41 illustratesan example of specifying actual edges between the routing graph nodes onlayer 3 based on the global preferred horizontal direction and the twolocal preferred diagonal directions of this layer.

Other embodiments, however, define (at 4315) the actual edges in therouting graph differently. For each potential edge, these embodimentsinitially identify an associated region, called the edge region. Theseembodiments then determine whether the potential edge should be anactual edge based on the proportion of edge region in which the locallypreferred direction matches the potential edge's direction.

FIG. 45 illustrates several examples of edge regions. Specifically, thisfigure illustrates a wiring layer that has been divided into a set ofGcells and four quadrants (i.e., four nodes) in each Gcell. The possibleedges in some embodiments between such nodes were described above inFIGS. 37-41. As mentioned above, the potential horizontal (H) edges arebetween SE-SW and NE-NW quadrants, and the potential vertical (V) edgesare between NW-SW and NE-SE quadrants. These potential Manhattan edgescome in two flavors: external (between quadrants in different Gcells)and internal (between quadrants in the same Gcell). The potentialdiagonal edges include 45° (D45) edges between SE-NW quadrants ofdifferent Gcells, and −45° (D135) edges between SW-NE quadrants ofdifferent Gcells.

FIG. 45 illustrates examples of six edge regions for six potentialedges. These six edge regions are (1) the edge region 4505 that isassociated with the internal horizontal edge 4510, (2) the edge region4515 that is associated with the external horizontal edge 4520, (3) theedge region 4525 that is associated with the internal vertical edge4530, (4) the edge region 4535 that is associated with the externalvertical edge 4540, (5) the edge region 4545 that is associated with theexternal D45 edge 4550, and (6) the edge region 4555 that is associatedwith the external D135 edge 4560.

For any potential edge of type H, V, D45, or D135 on each layer, someembodiments define f_(H), f_(V), f_(D45), or f_(D135), which are thefraction of the area in the edge's associated edge region on that layerat which the preferred direction matches the edge type. Some embodimentsuse the following rules to determine whether to specify an actual edgefor a potential edge, where f_(M) can be either f_(H) and f_(V), f_(D)can be f_(D45) or f_(D135):

-   -   Manhattan edge:        -   Define an actual edge if the f_(M) of an external potential            Manhattan edge (i.e., a Manhattan edge that connects two            nodes in two different Gcells) is greater than one third.        -   Define an actual edge if the f_(M) of an internal potential            Manhattan edge is greater than zero.    -   Diagonal edge:        -   Define an actual edge if the f_(D) of an external potential            diagonal edge (i.e., a diagonal edge that connects two nodes            in two different Gcells) is greater than one half.

Instead of using ⅓, 0, and ½ in the above-mentioned rules, otherembodiments might use other parameters. In some embodiments, the routerallows a designer to adjust these parameters.

It should be noted that the potential D45 and D135 edges across anyGcell boundary are associated with the same diamond-shaped stitchregion. To avoid generating unresolvable nonplanaraties, someembodiments at most define only one of these diagonal edges at any Gcellboundary. The proposed criterion, f_(D)>½, ensures only one diagonaledge is defined at any Gcell boundary. If a value less than ½ isselected, it would become possible for both edge directions to exceedthe threshold, in which case some embodiments define an edge only in thedirection with the greater fraction.

In the current global routing model, D45 and D135 routes use disjointsets of quadrants ({SE, NW} and {SW, NE}, respectively). To avoid everyroute between such regions having to via to another layer and back, someembodiments stitch adjacent quadrants together if they are touched byD45 and D135 stitches.

D. Edge Capacity Estimation

As mentioned above, some embodiments compute the capacity of edges inthe Groute graph that correspond to regions in the layout with only onepreferred direction based on existing capacity estimation techniques.U.S. Published patent application 2004-0098680 describes some of theexisting capacity estimation techniques.

These embodiments, however, compute the capacity of edges in the Groutegraph that correspond to regions in the layout with multiple differentpreferred directions based on novel capacity estimation techniques. Todefine the capacity along a particular edge that crosses a particularGcell boundary in a particular direction (e.g., a horizontal edgebetween two Gcells), some embodiments first identify a capacity tilethat is associated with the particular edge. In some embodiments, thecapacity tile for a particular edge is identical to the edge region thatwas used to determine whether to define the particular edge. Theabove-described FIG. 45 illustrated several examples of edge regions fordifferent types of external edges (where an external edge is an edgebetween two Gcells).

In other embodiments, however, the capacity tile associated with aparticular edge is different than the edge region of the particularedge. For instance, in some embodiments, the capacity tile is a squarethat is one Gcell wide and that is centered about the Gcell boundarycrossed by the particular edge. Two such capacity tiles are illustratedin FIG. 46. In these figures, the dark-outline squares depict the Gcells4605, 4610, 4615, and 4620, and dashed-outline squares represent thecapacity tiles 4625 and 4630. The capacity tiles are used for bothdiagonal and Manhattan directions.

In FIG. 46, the Gcell boundary between cells 4605 and 4610 is designatedas an E-W edge, while the Gcell boundary between cells 4615 and 4620 isdesignated as a N-S edge. In some embodiments, the possible planar edgesalong the E-W edge can be horizontal, 45°, and −45° directions, whilethe possible planar edges along the N-S edge can be vertical, 45°, and−45° directions.

After identifying the capacity tile associated with a particular edge,some embodiments pixelate the capacity tile. The pixelation operationdivides the capacity tile into numerous square tiles, and representseach square tile in terms of a pixel located at the tile's center. FIG.47 illustrates an example of pixelating a capacity tile 4700 intonumerous tiles with pixels at their centers. As shown in this figure,capacity tile 4700 includes two obstacles 4705 and 4710, and one region4750 with a 45° local preferred direction.

A pixel has three attributes: (1) Routing_Direction, which can be any ofH, V, D45, or D135, (2) blocked, which can be true or false to indicatewhether the pixel is free or blocked, and (3) half-blocked, which whenset indicates that the pixel is only half blocked (as opposed to fullblocked when the blocked field is true). The routing direction of apixel will be the routing direction at the pixel's location. If thepixel is on the edge/vertex of an LPDR, a precedence order (D0, D90,D45, D135) will determine the direction. One reason to have awell-defined precedence is to remove any dependency on LPDR-ordering orplatform. A pixel is flagged as a blocked pixel when a blockage (e.g.,an obstacle, pin, or a previously defined route) overlaps some portionof the tile associated with the pixel. However, a pixel will not bedesignated as a blocked pixel by a mere touch of a blockage. In someembodiments, there must be a non-zero overlap between the pixel's tileand a blockage before the pixel is deemed to be blocked.

The distance between the pixels will determine the accuracy of detectingnarrow single-track openings. Some embodiments define the pixel pitchequal to the track pitch. When the pixel pitch equals the track pitch,an opening up to 2.0 track-pitches might, depending on alignment withrespect to the pixels, get flagged as no-opening, as shown in FIG. 48.

After pixelating a capacity tile, some embodiments then perform a raytracing operation to quantify the capacity of the particular edge. Givena starting pixel from a starting set of pixels, a ray can be traced byjumping along the routing-direction to the next pixel, and then usingthe next pixel's routing-direction for the subsequent move and so on.The ray will end when it reaches an end pixel in an ending set ofpixels, or when it reaches a blocked edge or a previously used pixel. Ifthe ray fails to reach an end pixel, the ray will be discarded and allthe pixels that the ray used before failing will be freed up for use bya subsequent ray-traversal. The capacity of the edge is then determinedbased on the number of rays that reach the end set of pixels.

FIG. 47A illustrates an example of such ray tracing. In this example,the ray tracing is performed from a start pixel in the leftmost pixelcolumn to an end pixel in the rightmost pixel column. FIG. 47A presentstwo sets of pixels as blacked out to illustrate that they are blocked byobstacles 4705 and 4710 in the capacity tile 4700. Also, this figureillustrates a horizontal ray 4720 (at the top of the pixelated tile4700) that traverses across this tile completely in the horizontaldirection. It also illustrates three rays 4730, 4735, and 4740 thattraverse across the pixelated tile first in a horizontal direction, thenin a 45° direction, and finally in the horizontal direction. These rayschange from between the horizontal and 45° directions twice as theyenter and exit the region 4750 that has a 45° local preferred direction.Finally, FIG. 47A illustrates one ray 4745 that never reaches a pixel inthe rightmost pixel column. This rays gets blocked at pixel 4755 becausethis pixel's direction (which is the LPD of the LPDR 4750) would requirethe ray to use pixel 4760, which was previously used by ray 4720.

In some cases, ray tracing might not give the same result if the ray isstarted from the end point. FIG. 47B illustrates an example that isidentical to the example illustrated in FIG. 47A, except that in FIG.47B the ray tracing is performed from right to left. As shown in FIG.47B, this ray tracing results in only one ray reaching its destination.However, for LPDRs spanning across many Gcells, such differences in theresult of the ray tracing are expected to be a small percentage.Notwithstanding, some embodiments address this difference by performingboth ray tracing operations (i.e., by once starting a ray tracingoperation from the start points and once starting a ray tracingoperation from the end points) and taking the bigger value produce bythe two operations. Other embodiments might take the smaller valueproduced by the two operations.

a. Special Consideration for Diagonal Movements Along Pixels

A diagonal movement from a first pixel to a second pixel require notonly that the second pixel be free but also requires the two pixels thatneighbor both the first and second pixels to be free. This isillustrated by the example illustrated in FIG. 49. This figureillustrates a move from pixel 4905 to 4910 in the 45° direction. Forsuch a move to be possible, the pixels 4910, 4915, and 4920 have to befree. Such a move would result in the marking of pixel 4910 as beingblocked. It would also result in the marking of pixels 4915 and 4920 asbeing blocked as the destination of a ray for all directions and asbeing “half used” for the 45° direction.

A half used pixel for a particular diagonal direction is a pixel thatcannot serve as the destination of a ray in the particular diagonaldirection but can serve as the neighboring pixel to two pixels that areconnected in the 45° direction. Pixel 5005 in FIG. 50 is an example of apixel that has two half uses. Specifically, half of this pixel is usedby wire 5010, while the other half of this pixel is used by wire 5015.

b. The Direction to Move Along When a Change of Direction Occurs

While ray tracing, it might be possible to move in two directionswhenever there is a change of direction. For instance, as shown in FIG.51, a horizontally moving ray might reach a pixel that is in a LPDR thathas a −45° LPD. In such a case, the next move of the ray can be in the135° direction or in the collinear −45° direction.

In such circumstances, some embodiments will move in the new directionthat is “Forward” to the existing direction. Mathematically this is thedirection that will result in a positive number when its dot-productwith the previous direction is taken. Alternatively, Table 1 belowidentifies the direction to select whenever the direction of theprevious pixel and the current pixel along the ray differs. TABLE 1Current Previous D0 D45 D90 D135 D0 — D45 Orthogonal D315 D45 D0 — D90Orthogonal D90 Orthogonal D45 — D135 D135 D180 Orthogonal D90 — D180 -NA - D225 Orthogonal D135 D225 D180 - NA - D270 Orthogonal D270Orthogonal D225 - NA - D315 D315 D0 Orthogonal D270 - NA -

For orthogonal routing-direction, some embodiments determine the movedirection in a way such that two criteria are met. First, to match thebehavior of the detailed router, there is no permeability if theorthogonal regions are separated by an edge that is perpendicular (orparallel) to any of the two directions. One such example is illustratedin FIG. 52. This figure illustrates a horizontal ray 5205 thatterminates when it reaches a LPDR 5210 with a vertical LPD, since theboundary of the LPDR region is orthogonal to the horizontal directionand parallel to the vertical direction.

Second, the direction is away from the existing region, so that the movecannot re-enter the region, and then zig-zag along the border. FIG. 53illustrates an example of this criteria. This figure illustrates ahorizontal ray 5305 that enters an LPDR region 5310 with a vertical LPD.Here, the LPDRs boundary reached by the ray is neither parallel nororthogonal to either direction. Hence, the ray 5305 can continue throughthe LPDR along its LPD. In this example, the ray moves up in the 90°direction instead of down in the −90° direction since moving down wouldresult in undesirable zig-zag along the LPDR boundary, as shown in FIG.53.

The pseudo code that some embodiments follow to implement these twocriteria for a ray that is going from a first pixel with a horizontaldirection to a second pixel with a vertical direction is as follows: Ifthe pixel below the first pixel has a horizontal direction { move-up//connect-to-top } else if the pixel above the first pixel has ahorizontal direction { move down //connect-to-bot } else { do notconnect }

For D45-D135 moves, the “diagonal move” criteria is more relaxed at thetransition point, as described above. Otherwise the diagonal moves wouldnot be possible.

c. Starting Points for the Rays

As mentioned above, a ray tracing operation will be performed afterpixellating a capacity tile. The starting point of the rays will bedependent on the planar edges that are available across the Gcellboundary associated with the capacity type.

Each edge-type will have a corresponding set of start and end-pointsalong the tile boundary. Union of the start-points for all the edgesavailable along the Gcell boundary will be the start points for theentire outline. The end-points will be chosen in a similar fashion. FIG.54 provides an illustration of eight sections Ctn1, Ctn2, Cts1, Cts2,Ctw1, Ctw2, Cte1, and Cte2 along the boundary of a tile that defineeight different sets of pixels that can server as start and end pointsfor different ray tracing operations. (The convention used for namingthese eight sections is capacity tile section name:ct<directionChar><num>.) For instance, Table 2 below provides the startand end-points for each ray tracing operation for each edge-type. TABLE2 Start Pixels are Pixels End Pixels are Pixels Edge Type Along BoundaryEdges Along Boundary Edges D0 Ctw1, ctw2 Cte1, cte2 D45 Ctw1, cts1 Ctn2,cte2 D90 Cts1, cts2 Ctn1, ctn2 D135 Ctn1, ctw2 Cte1, cts2

The rays will begin from the pixels along the starting edges. The orderof selecting pixels will be anti-clockwise beginning with the ctn1 edge.

Once a ray is able to reach its end-target, its orientation will bedetermined based on its two end-points. The ray will be classified bythe X-angle (D0, D45, D90 or D135) closest to the orientation. If thestitch to which the ray gets classified is not available, the ray willbe discarded and all the pixels will be freed for any subsequenttraversal. The capacity along an edge is the total number of rays thatsuccessfully traverse from “start” to “end”.

FIG. 55 illustrates the start and end edges for the 45° and 135°directions. As shown in this figure, the start and end edges are theedges of the locus of the third quadrant (Q3) of the capacity tilemoving to the tile's first quadrant (Q1) for 45° edge, and the locus oftile second quadrant (Q2) moving to the tile's fourth quadrant (Q4) for135° edge.

d. Obstructions

Big Manhattan obstructions need special handling in diagonal regions.Otherwise, the capacity that is calculated can allow routes through suchobstructions. Hence, in some embodiments, if a Manhattan obstructionintersects a capacity tile that has a planar diagonal edge, the part ofthe obstruction that overlaps the extended region (and outside the tile)will be projected onto the tile outline. The projection onto the outlinewill block the corresponding start/end pixels. The projection willverify/check for the pixels along the path being diagonal. If a pixel isnot diagonal, the projection ray will stop.

FIGS. 56A and 56B present two examples that illustrate this. In bothexamples, there is a capacity tile 5605 for a vertical boundary 5610between two Gcells 5615 and 5620. Also, in both examples, a horizontalaligned obstruction 5625 overlaps the capacity tile 5605 and extendsbeyond this tile in both directions. FIG. 56A illustrates projecting theextension of the obstruction past the tile on the right hand side ontothe capacity tile section Cte2 in the −135° direction. FIG. 56Billustrates projecting the extension of the obstruction past the tile onthe left hand side onto the capacity tile section Ctw2 in the −45°direction. These projections shown that the obstruction 5625 completelyblocks the pixels on the two sections Cte2 and Ctw2 for the 45° and 135°directions respectively.

e. Alternatives

Whenever a Manhattan and a diagonal region are separated by aManhattan-oriented border, the theoretical limit for transition is 70%.The above-described model, models this limit to be 50%. The detailedrouter might be closer to 70% in transition percentage and hence theabove-described model is somewhat conservative.

Choosing a pixel-pitch of half the track pitch would achieve theManhattan-diagonal transition percentage much closer to the theoreticallimit (70%). But more processing would be required to achieve thedetailed router track behavior (implicit when pixel-pitch is equal totrack-pitch). Some embodiments do not do this because of a 4-timesincrease in ray-tracing work and pixel-memory consumption. Memoryconsumption could, however, be controlled by not storing directions forpixels. These could be determined during ray-tracing (for the entireGcell at one-go). For faster runtimes, a bottom up search mechanism fora KD tree could be used. This would significantly increased theimplementation time.

Also, the above-described model has just one pixel-grid per direction.Other embodiments create a pixel grid for each direction, and definetransition zones (around LPD outlines) to move from one grid to another.

IV. Computer System

FIG. 57 conceptually illustrates a computer system with which someembodiment of the invention are implemented. Computer system 5700includes a bus 5705, a processor 5710, a system memory 5715, a read-onlymemory 5720, a permanent storage device 5725, input devices 5730, andoutput devices 3035.

The bus 5705 collectively represents all system, peripheral, and chipsetbuses that support communication among internal devices of the computersystem 5700. For instance, the bus 5705 communicatively connects theprocessor 5710 with the read-only memory 5720, the system memory 5715,and the permanent storage device 5725.

From these various memory units, the processor 5710 retrievesinstructions to execute and data to process in order to execute theprocesses of the invention. The read-only-memory (ROM) 5720 storesstatic data and instructions that are needed by the processor 5710 andother modules of the computer system. The permanent storage device 5725,on the other hand, is a read-and-write memory device. This device is anon-volatile memory unit that stores instruction and data even when thecomputer system 5700 is off. Some embodiments of the invention use amass-storage device (such as a magnetic or optical disk and itscorresponding disk drive) as the permanent storage device 5725. Otherembodiments use a removable storage device (such as a floppy disk orzip® disk, and its corresponding disk drive) as the permanent storagedevice.

Like the permanent storage device 5725, the system memory 5715 is aread-and-write memory device. However, unlike storage device 5725, thesystem memory is a volatile read-and-write memory, such as a randomaccess memory. The system memory stores some of the instructions anddata that the processor needs at runtime. In some embodiments, theinvention's processes are stored in the system memory 5715, thepermanent storage device 5725, and/or the read-only memory 5720.

The bus 5705 also connects to the input and output devices 5730 and5735. The input devices enable the user to communicate information andselect commands to the computer system. The input devices 5730 includealphanumeric keyboards and cursor-controllers. The output devices 5735display images generated by the computer system. For instance, thesedevices display IC design layouts. The output devices include printersand display devices, such as cathode ray tubes (CRT) or liquid crystaldisplays (LCD). Finally, as shown in FIG. 57, bus 5705 also couplescomputer 5700 to a network 5757 through a network adapter (not shown).In this manner, the computer can be a part of a network of computers(such as a local area network (“LAN”), a wide area network (“WAN”), oran Intranet) or a network of networks (such as the Internet). Any or allof the components of computer system 5700 may be used in conjunctionwith the invention. However, one of ordinary skill in the art willappreciate that any other system configuration may also be used inconjunction with the invention.

While the invention has been described with reference to numerousspecific details, one of ordinary skill in the art will recognize thatthe invention can be embodied in other specific forms without departingfrom the spirit of the invention. For instance, some embodiments definea crown boundary between an LPDR with a Manhattan LPD (e.g., ahorizontal direction) and an LPDR with a non-Manhattan LPD (e.g., a 45°diagonal direction) in terms of an angle that is between the Manhattanand non-Manhattan directions (e.g., a 22.5° direction). Thus, one ofordinary skill in the art would understand that the invention is not tobe limited by the foregoing illustrative details, but rather is to bedefined by the appended claims.

1. A method of routing comprising: a) defining at least one wiring layerthat has at least two regions with different local preferred wiringdirections; b) using the differing local preferred wiring directions todefine a global route on the wiring layer.
 2. The method of claim 1,wherein the two regions are a first region with a first local preferredwiring direction, and a second region with a second local preferredwiring direction, wherein the global route traverses the first regionalong the first local preferred wiring direction and traverses thesecond region along the second local preferred wiring direction.
 3. Fora design layout that has multiple wiring layers, wherein at least oneparticular layer has at least two regions with different local preferredwiring directions, a method of defining global routes, the methodcomprising: a) defining a routing graph for specifying global routes,said routing graph having a plurality of edges for defining routes; b)defining the capacities of a plurality of edges based on the differentlocal preferred wiring directions of the two regions; c) using thedefined capacities of the edges to define global routes.
 4. The methodof claim 3 further comprising: a) identifying a plurality of potentialedges for use by a route; b) for each particular identified edge,determining whether the particular edge has sufficient availablecapacity for another route to use the edge; c) when the particular edgehas sufficient available capacity, using the particular edge to definethe route.
 5. The method of claim 4, wherein when the particular edgedoes not have sufficient available capacity, foregoing the use of theparticular edge to define the route.
 6. The method of claim 4, whereinidentifying the plurality of potential edges comprises performing a pathsearch that identifies a plurality of path expansions across theplurality of potential edges.
 7. The method of claim 6, whereinperforming a path search comprises: a) identifying a first expansionacross a first edge, wherein the first edge is associated with a set ofregions in the particular layer; b) determining whether the firstexpansion across the first edge is along a set of local preferreddirections of the set of regions associated with the first edge; c) whenthe first expansion is along the set of local preferred directions,determining whether the first edge has sufficient remaining capacity foranother expansion.
 8. The method of claim 6 further comprising: a)before the path search, identifying a set of potential directions forexpansions across each of a plurality of edges based on the localpreferred directions; b) using the identified potential directions toidentify expansions during the path search.
 9. The method of claim 6,wherein each edge has an associated direction, the method furthercomprising: a) before the path search, using the local preferreddirections to identify edges in the routing graph; b) using theidentified edges to identify path expansions during the path search. 10.The method of claim 4 further comprising reducing a particular edge'scapacity after using the particular edge to define a route, in order toaccount for the edge's use by the route.
 11. The method of claim 3,wherein each edge is associated with at least one area in the layout,wherein defining the capacity of an edge comprises: a) dividing the areainto a plurality of sub-regions; b) generating lines that start at oneset of sub-regions and traverse across the area based on the preferredwiring directions of the sub-regions; c) identifying the capacity of theedge based on the number of generated lines that terminate on anotherset of sub-regions.
 12. The method of claim 3, wherein each edge isassociated with at least one area in the layout, wherein defining thecapacity of an edge comprises: a) determining whether the areaassociated with the edge covers a boundary between two different regionswith two different local preferred directions; b) when the edge coversthe boundary between the two different regions, computing the estimatednumber of wire tracks that are incident on that boundary in one regionand the estimated number of wire tracks that are incident on thatboundary in the other region; c) using the estimated numbers to definethe capacity of the edge.
 13. The method of claim 3, wherein each of aplurality of layer has at least two regions with different localpreferred wiring directions, the method further comprising: a) definingthe capacities of a plurality of edges based on the different localpreferred wiring directions of the different regions on the differentlayers; b) using the defined capacities of the edges to define globalroutes.
 14. The method of claim 3, wherein the design layout is an ICdesign layout.
 15. A computer readable medium that stores a computerprogram for routing a design layout that has multiple wiring layers,wherein at least one particular layer has at least two regions withdifferent local preferred wiring directions, the program comprising setsof instructions for: a) defining a routing graph for specifying globalroutes, said routing graph having a plurality of edges for definingroutes; b) defining the capacities of a plurality of edges based on thedifferent local preferred wiring directions of the two regions; c) usingthe defined capacities of the edges to define global routes.
 16. Thecomputer readable medium of claim 15, wherein the computer programfurther comprises sets of instructions for: a) identifying a pluralityof potential edges for use by a route; b) for each particular identifiededge, determining whether the particular edge has sufficient availablecapacity for another route to use the edge; c) when the particular edgehas sufficient available capacity, using the particular edge to definethe route.
 17. The computer readable medium of claim 16, wherein whenthe particular edge does not have sufficient available capacity,foregoing the use of the particular edge to define the route.
 18. Thecomputer readable medium of claim 16, wherein the set of instructionsfor identifying the plurality of potential edges comprises a set ofinstructions for performing a path search that identifies a plurality ofpath expansions across the plurality of potential edges.
 19. Thecomputer readable medium of claim 16, wherein the program furthercomprises a set of instructions for reducing a particular edge'scapacity after using the particular edge to define a route, in order toaccount for the edge's use by the route.
 20. The computer readablemedium of claim 15, wherein each edge is associated with at least onearea in the layout, wherein the set of instructions for defining thecapacity of an edge comprises a set of instructions for: a) dividing thearea into a plurality of sub-regions; b) generating lines that start atone set of sub-regions and traverse across the area based on thepreferred wiring directions of the sub-regions; c) identifying thecapacity of the edge based on the number of generated lines thatterminate on another set of sub-regions.
 21. The computer readablemedium of claim 15, wherein each edge is associated with at least onearea in the layout, wherein the set of instructions for defining thecapacity of an edge comprises sets of instructions for: a) determiningwhether the area associated with the edge covers a boundary between twodifferent regions with two different local preferred directions; b) whenthe edge covers the boundary between the two different regions,computing the estimated number of wire tracks that are incident on thatboundary in one region and the estimated number of wire tracks that areincident on that boundary in the other region; c) using the estimatednumbers to define the capacity of the edge.
 22. The computer readablemedium of claim 15, wherein each of a plurality of layer has at leasttwo regions with different local preferred wiring directions, theprogram further comprises sets of instructions for: a) defining thecapacities of a plurality of edges based on the different localpreferred wiring directions of the different layers; b) using thedefined capacities of the edges to define global routes.